shell: handle $((NUM++...) like bash does. Closes 10706

function                                             old     new   delta
evaluate_string                                      680     729     +49

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/ash_test/ash-arith/arith.right

@@ -126,6 +126,10 @@ ghi
 ./arith2.sub: line 5: arithmetic syntax error
 5 5
 1 1
+6 6
+2 2
+3 3
+1 1
 4 4
 0 0
 ./arith2.sub: line 42: arithmetic syntax error